1. Field of the Invention
The invention is directed to an actuating device for a friction clutch which is arranged in the drivetrain of a motor vehicle, especially of a commercial vehicle, in a housing cover between an internal combustion engine and a transmission. The invention relates further to an actuating device which comprises a release bearing arrangement, which is movable substantially coaxially to the friction clutch for actuating the friction clutch, and a positioning servo arrangement with a pressure medium power cylinder arrangement which acts on the release bearing arrangement and which can be actuated by means of a control valve connected with a pressure medium source depending on a control quantity representing a reference position and depending on an actual quantity representing the axial position of the release bearing arrangement.
2. Description of the Prior Art
An actuating device of this kind is known, for example, from German DE 33 21 578 C2. This known actuating device has a vacuum servo booster or power assist as a positioning servo arrangement. The servo power assist, which is constructed in the manner of a vacuum brake power assist, is constructed integral with a pneumatic power cylinder and integral with a control valve, and is arranged outside of the housing cover. Two work chambers of the pneumatic power cylinder are separated from one another by a piston which is guided so as to be movable axially and a resilient diaphragm. One work chamber, which is constructed as a vacuum chamber, is connected to an intake line system of the internal combustion engine. The other work chamber, which serves as a control chamber, can communicate by means of the control valve either with the vacuum chamber or, via a pressure compensation port, with the atmosphere. The actuation of the vacuum servo power assist is effected by means of a control rod which acts on the valve body of the control valve and which is axially displaceable via a cam driven by en electric motor. As a result, an axial displacement of the control rod occurs, the control valve is switched in such a way that the piston follows the movement of the control rod in a power-assisted manner. By way of a hydraulic master cylinder, the movement of the piston acts on a slave cylinder which is connected with the master cylinder and arranged outside of the housing cover and acts, in turn, on a release fork associated with the release bearing arrangement. An emergency actuation of the clutch is possible when the internal combustion engine is stopped, that is, in the absence of vacuum pressure, in that the control rod can act directly on the piston via the valve body of the control valve.
The control valve has a valve body which cooperates with a resilient valve seat. A connection duct is provided in the valve seat, the connection between the control chamber and the surrounding air being effected via this connection duct, insofar as the valve body is not pressed against the resilient valve seat, in order to close the connection duct through the valve body. A further connection duct connects the control chamber with the vacuum chamber.
Further, a known actuating device of the type mentioned above has a pneumatic force cylinder as a pressure medium power cylinder arrangement. The actuating device is arranged outside of the housing cover as an integral unit comprising the pneumatic power cylinder, a hydraulic slave cylinder and the control valve. The piston of the pneumatic power cylinder is arranged at a rod element forming the piston of the hydraulic slave cylinder, and the rod element is connected with a tappet which extends into the interior of the housing cover and acts at a release fork associated with the release bearing arrangement. A master cylinder which can be actuated by the clutch pedal, and a control unit of the control valve are connected to the hydraulic slave cylinder. The control valve controls the supply of compressed air to the pneumatic power cylinder depending on the hydraulic pressure present at the control input and controls the release of air from the pneumatic power cylinder, respectively, so that a given hydraulic pressure determined by a pressure spring arrangement is adjusted at the control input. The hydraulic slave cylinder serves as a measurement cylinder which detects the position of the rod element and accordingly indirectly detects the position of the release bearing arrangement. When the master cylinder is actuated, forces are exerted directly on the rod element, and accordingly on the release bearing arrangement, via the hydraulic slave cylinder serving as measurement cylinder, in addition to the actuating forces of the pneumatic power cylinder due to the supply of compressed air.